It is known that a polymer having an uneven distribution of a polymerization rate, i.e., a polymer with its polymerization rate varied depending on location within the polymer (hereinafter referred to as polymerization rate-distributed polymer) can be obtained by impregnating a polymerizable monomer into a substrate with a concentration gradient, followed by polymerization. According to this technique, however, the impregnated monomer concentration shows a diffusion distribution having the highest concentration in the periphery of the substrate, and it is difficult to provide a highly concentrated region at a prescribed position, e.g., inside the substrate. That is, the process has a narrow freedom of polymerization rate distribution.
A modification of the above-described process has been proposed, in which part of the surface of a substrate is covered with an impermeable mask to control the site where a polymerizable monomer is impregnated. However, this modification makes no difference in terms of diffusion distribution from the site of impregnation toward the surroundings, and the freedom of polymerization rate distribution is still limited. Therefore, whichever process one may choose, a concentration distribution is determined by the type of substrate and polymerizable monomer and environmental conditions. The processes are difficult to control, freedom of the possible pattern of polymerization rate distribution is extremely narrow, and there are difficulties in controlling distribution mode and in freely designing polymerization rate-distributed regions.
On the other hand, it has been proposed to expose a photopolymerizable compound or a photo-crosslinkable compound to light through an optical mask having a binary pattern having openings at varied intervals to produce a polymerization rate-distributed polymer or a crosslinked article with its crosslinking rate unevenly distributed (hereinafter referred to as a crosslinking rate-distributed article) based on the areas of the irradiated portions (exposure portions) and unirradiated portions (non-exposure portions). However, this process is disadvantageous in that preparation of such an optical mask requires much time and labor, an optical mask must be prepared for every pattern of proposed distribution, and incident light is diffracted by the optical mask, resulting in poor reproduction precision of distribution. In particular, the diffraction of light becomes greater as the pattern is made finer. Therefore, the process lacks reproducibility in forming fine regions needing highly precise control as in the formation of microlenses.